Here are some relevant blockquotes of Bostrom's reasoning on brain-computer interfaces, from Superintelligence chapter 2:

It is sometimes proposed that direct brain–computer interfaces, particularly implants, could enable humans to exploit the fortes of digital computing—perfect recall, speedy and accurate arithmetic calculation, and high-bandwidth data transmission—enabling the resulting hybrid system to radically outperform the unaugmented brain.64 But although the possibility of direct connections between human brains and computers has been demonstrated, it seems unlikely that such interfaces will be widely used as enhancements any time soon.65

To begin with, there are significant risks of medical complications—including infections, electrode displacement, hemorrhage, and cognitive decline—when implanting electrodes in the brain. ... One study of Parkinson patients who had received deep brain implants showed reductions in verbal fluency, selective attention, color naming, and verbal memory compared with controls. Treated subjects also reported more cognitive complaints.66 Such risks and side effects might be tolerable if the procedure is used to alleviate severe disability. But in order for healthy subjects to volunteer themselves for neurosurgery, there would have to be some very substantial enhancement of normal functionality to be gained.

Futhermore:

enhancement is likely to be far more difficult than therapy. Patients who suffer from paralysis might benefit from an implant that replaces their severed nerves or activates spinal motion pattern generators.67 Patients who are deaf or blind might benefit from artificial cochleae and retinas.68 Patients with Parkinson’s disease or chronic pain might benefit from deep brain stimulation that excites or inhibits activity in a particular area of the brain.69 What seems far more difficult to achieve is a high-bandwidth direct interaction between brain and computer to provide substantial increases in intelligence of a form that could not be more readily attained by other means. Most of the potential benefits that brain implants could provide in healthy subjects could be obtained at far less risk, expense, and inconvenience by using our regular motor and sensory organs to interact with computers located outside of our bodies. We do not need to plug a fiber optic cable into our brains in order to access the Internet.

Not only can the human retina transmit data at an impressive rate of nearly 10 million bits per second, but it comes pre-packaged with a massive amount of dedicated wetware, the visual cortex, that is highly adapted to extracting meaning from this information torrent and to interfacing with other brain areas for further processing.70 Even if there were an easy way of pumping more information into our brains, the extra data inflow would do little to increase the rate at which we think and learn unless all the neural machinery necessary for making sense of the data were similarly upgraded. Since this includes almost all of the brain, what would really be needed is a “whole brain prosthesis–—which is just another way of saying artificial general intelligence. Yet if one had a human-level AI, one could dispense with neurosurgery: a computer might as well have a metal casing as one of bone. So this limiting case just takes us back to the AI path, which we have already examined.